Brain-Steroid Interactions and the Control of Aggressiveness in Birds. In: Müller EE, Macleod RM, editors. Neuroendocrine Perspectives. New York: Springer; 1991. pp. 1-43. [DOI: 10.1007/978-1-4612-3146-2_1]

New biosynthesis and biological actions of avian neurosteroids

De novo neurosteroidogenesis from cholesterol occurs in the brain of various avian species. However, the biosynthetic pathways leading to the formation of neurosteroids are still not completely elucidated. We have recently found that the avian brain produces 7α-hydroxypregnenolone, a novel bioactive neurosteroid that stimulates locomotor activity. Until recently, it was believed that neurosteroids are produced in neurons and glial cells in the central and peripheral nervous systems. However, our recent studies on birds have demonstrated that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol. 7α-Hydroxypregnenolone is a major pineal neurosteroid that stimulates locomotor activity of juvenile birds, connecting light-induced gene expression with locomotion. The other major pineal neurosteroid allopregnanolone is involved in Purkinje cell survival during development. This paper highlights new aspects of neurosteroid synthesis and actions in birds.

Androgen synthesis in a songbird: a study of cyp17 (17alpha-hydroxylase/C17,20-lyase) activity in the zebra finch

Androgens and estrogens influence the maturation and function of numerous tissues in both male and female birds, especially the brains of the oscine songbirds. Although there exist a very large number of studies that have investigated circulating sex steroids in many species of wild and captive-held songbirds, there remain a significant number of questions about the sites of synthesis of the active steroids that act on the songbird brain. Estrogens are derived from androgen. Thus, the synthesis of androgen itself is critical for both androgen- and estrogen-dependent actions in both male and female songbirds. Therefore, we have undertaken studies of the enzyme 17alpha-hydroxylase/C17,20-lyase (Cyp17), the enzyme responsible for the synthesis of androgens from their progestin or pregnane precursors via their 17alpha-hydroxy intermediates. Here we have characterized optimal conditions for measuring Cyp17 in gonads of adult zebra finches via the conversion of tritiated [3H]progesterone into 17alpha-hydroxy P (17alpha-hydroxylase activity) and androstenedione and testosterone (C17,20-lyase) activity. Cyp17 activity is abundant in testis, with lesser amounts in ovary. Low levels of Cyp17 activity were also detected in male adrenals, but not in any other tissue, including brain. Testicular Cyp17 activity is readily inhibited in vitro by ketoconazole, a specific Cyp17 inhibitor. Ketoconazole works less well in vivo. In males castrated and/or treated with fadrozole, an inhibitor of aromatase, we detected no extragonadal sites of Cyp17 activity, although fadrozole appeared to increase circulating androgens in both castrated and intact males. Thus, we still do not know the site of androgen synthesis in these males. Further studies of Cyp17 will be useful in understanding more about the mechanisms of androgen delivery to neural circuits in adult and developing songbirds.

The passerine hippocampus is a site of high aromatase: inter- and intraspecies comparisons

The vertebrate hippocampus (HP) plays a critical role in the organization of memory. Estrogens alter synaptic morphology and function in the mammalian HP and may potentiate memory performance. Previous studies suggest that the songbird HP itself is a site of significant aromatase expression, intimating that local estrogen synthesis may provide a source of this steroid to estrogen-sensitive neural circuits. To explore the potential role of local estrogen synthesis on HP structure and function, we have characterized aromatase message and activity in the zebra finch HP. Toward this end we have compared (a) HP aromatase mRNA with that at other neural loci, (b) HP aromatase activity between adults of both sexes, and (c) HP and hypothalamic preoptic area (HPOA) aromatase activity among songbirds and nonsongbirds. Finally we asked whether aromatase activity was intrinsic to the HP by maintaining it in culture, isolated from the rest of the telencephalon. The HP of every songbird studied expresses aromatase, with comparable levels across sexes. Notably, aromatase activity was found at higher levels in the songbird HP than in the HPOA. In both nonsongbird species investigated, however, HP aromatase activity was undetectable under identical assay conditions. Additionally, the developing songbird HP continues to express aromatase when cultured in isolation from the rest of the telencephalon. The data suggest that HP aromatase is characteristic of passeriformes and, as in the HPOA, may represent a mechanism whereby estrogen is made available to neural circuits. Passerines may prove invaluable animal models for investigations of the estrogenic modulation of HP structure and function.

Sexual differentiation of avian brain and behavior: current views on gonadal hormone-dependent and independent mechanisms

Gonadal hormones are known to act during development to establish permanent sex differences in the anatomy and function of the vertebrate brain. They also act on the adult brain to activate reproductive behaviors. However, there are wide gaps in our understanding of how sexually dimorphic neural circuits translate into sex differences in behavior and other CNS functions. Moreover, not all sexually dimorphic properties of the adult brain can be attributed to known effects of gonadal hormones during development or adulthood, and factors other than gonadal steroids may contribute to these sex differences. This paper reviews sexual differentiation and the role of gonadal steroids and non-gonadal factors on sexually dimorphic development of the avian brain.

Activities of aromatase and 3beta-hydroxysteroid dehydrogenase/delta4-delta5 isomerase in whole organ cultures of tissues from developing zebra finches

The hormonal basis for masculine song development in the zebra finch remains unidentified. To understand how steroids are differentially supplied to the brains of males and females to cause sexually dimorphic development of this behavior, we have studied the steroidogenic capability of zebra finch tissues during early development (1 to 8 days posthatching). Here, we report on the use of cultures of whole gonads, adrenals, and telencephalons to measure the activities of two steroidogenic enzymes: aromatase, the enzyme that catalyzes the conversion of androgen to estrogen, and 3beta-hydroxysteroid dehydrogenase/delta4-delta5 isomerase (3beta-HSD), the enzyme that converts pregnenolone into progesterone. We also examined the effect of cAMP on aromatase activity in these tissues as this intracellular second messenger has been shown previously to regulate aromatase in both central and peripheral tissues of other species. In untreated cultures, aromatase was detected at the highest levels in male and female telencephalon and in ovary. Dibutyryl (dB)-cAMP had no significant effect on aromatase activity in any tissue. However, after dB-cAMP treatment, estrogens were regularly detected in cultures of whole testes. Although this activity was relatively low when compared to total activity found in other tissues, due to the small size of the testes at this age of development, the specific activity (per milligram of protein) might be high enough to produce some estrogen. Adrenal aromatase was unconfirmed in the presence or absence of cAMP. 3Beta-HSD activity was undetected in brain but was detected in gonads and adrenals from all birds. There were no significant differences in gonadal or adrenal 3beta-HSD activity between males and females. Although these data present the first evidence for testicular aromatase in the zebra finch, they provide no evidence to support a mechanism to generate a greater estrogenic signal in male zebra finches after hatching.

Estrogen synthesis and secretion in the brown-headed cowbird (Molothrus ater)

Estrogens exert profound effects on vertebrate physiology and behavior. In most vertebrates, including birds, estrogens derived from ovarian tissue circulate at high levels during discrete periods of reproductive activity, and estrogen levels in males are low. In some songbirds (Passeriformes) plasma estrogens are high in both males and females. In the zebra finch, aromatase (estrogen-synthetase) is expressed abundantly in several regions of the male and female telencephalon and contributes to peripheral estrogen titers. To determine if this pattern of neural aromatase and estrogen synthesis is found in other songbirds, we have examined the patterns of estrogen synthesis in various tissues of another songbird, the common North American cowbird (Molothrus ater). Radiolabeled aromatizable androgenic substrate was injected in vivo or provided in vitro to telencephalic and gonadal tissue from adult male and female cowbirds. Estrogenic products were assayed in blood from the carotid artery and jugular vein, and in the telencephalon, ovary, and testes. Additionally, the presence of aromatase mRNA was studied in the brain using in situ hybridization. Radiolabeled androgenic substrate, injected in vivo, was readily converted to estrogens with higher amounts in the jugular compared to carotid blood, suggesting that the brain contains relatively high levels of aromatase. Further, radiolabeled androgens, provided in vitro to telencephalic, ovarian, and testicular tissue, resulted in the formation of radiolabeled estrogens. Aromatase mRNA is distributed widely in several areas of the cowbird telencephalon including the hippocampus, caudomedial neostriatum (including Field L), and nucleus taeniae. This pattern of neural aromatase expression resembles what we found previously in the zebra finch. Telencephalic aromatase may be characteristic of passerine songbirds and may function to provide local estrogenic cues to estrogen-sensitive neural loci, and/or contribute to peripheral estrogen titers in male and female songbirds.

An atlas of aromatase mRNA expression in the zebra finch brain

Neural conversion of androgen to estrogen by aromatase is an important step in the development and expression of masculine behavior in mammals and birds. In contrast to the low telencephalic levels of aromatase in adult mammals and nonsongbirds, the zebra finch telencephalon possesses high aromatase activity. This study maps, by in situ hybridization, cells that express aromatase mRNA in the adult zebra finch telencephalon, diencephalon, midbrain, and pons. High aromatase mRNA expression was observed in the caudal neostriatum, limbic archistriatum, and hypothalamus. The hippocampus, parahippocampal area, and hyperstriatum accessorium contained cells expressing moderate amounts of aromatase message. Weakly labeled cells were found in the rostral neostriatum, lobus parolfactorius, and mesencephalic reticular formation. These findings are consistent with aromatase activity measurements of zebra finch tissue and document with anatomical precision both the widespread expression of aromatase mRNA in the brain and novel sites of brain aromatase. This study identifies the caudal neostriatum as a major site of telencephalic aromatase. A previous survey (Gahr et al., 1993: J. Comp. Neurol. 327:112-122) of several avian species found that the presence of estrogen receptors in parts of the caudal neostriatum is unique to songbirds, which are the only birds to possess the elaborated telencephalic song system. Together, these findings suggest that the heightened estrogen synthesis and estrogen sensitivity of the passerine caudal neostriatum may have some functional relation with the telencephalic circuits responsible for song.

Aromatase and 5 beta-reductase activity in cultures of developing zebra finch brain: an investigation of sex and regional differences

Estrogen treatment of hatchling female zebra finches causes the masculine development of singing behavior and of the telencephalic brain regions involved in the control of song. However, early estrogen treatment of males also blocks masculine development of copulatory behavior, presumably controlled by diencephalic regions. In an effort to determine whether the differences in estrogen action are related to sex and regional differences in androgen metabolism (estrogen synthesis or androgen inactivation), we measured aromatase and 5 beta-reductase activity in dissociated-cell cultures made separately from the telencephalon, diencephalon, and also cerebellum of hatchling zebra finches under a variety of conditions. Cultures from all three brain regions express high levels of aromatase and 5 beta-reductase activity. Comparisons between telencephalic and diencephalic cultures of the activity and kinetics of aromatase suggest that the telencephalic cultures convert androgen to estrogen more efficiently than diencephalic cultures, which might be important in the differential action of estrogen in the two brain regions. However, the activity of neither aromatase nor 5 beta-reductase was significantly different between the sexes in either telencephalic or diencephalic cultures. Thus, comparisons between the sexes do not support the idea that differences in posthatching aromatase or 5 beta-reductase activity account for the pattern of sexual differentiation of the song and copulatory systems.

5 beta-reductase and other androgen-metabolizing enzymes in primary cultures of developing zebra finch telencephalon

Enzymes in the avian brain irreversibly catalyze the conversion of androgens into either active metabolites (aromatase and 5 alpha-reductase) or inactive metabolites (5 beta-reductase), 5 beta-reductase is thought to influence the formation of active metabolites by reducing the concentration of androgenic substrate available for these reactions. However, because these enzymes have different regional, cellular and subcellular distributions in brain, the traditional method to measure enzyme activity in brain homogenates may be inaccurate because of artificial mixing of enzymes. Recently, we have prepared primary cell cultures from the telencephalons of developing zebra finches. Cell cultures offer the advantage that enzyme activity can be measured in live cells in which the relative distribution of enzymes may more closely reflect that found in vivo. We have previously reported that aromatase is expressed at high levels in these cultures, and that it is active in both neurons and in glia. Here we report on the activities of 5 alpha- and 5 beta-reductase in these cell cultures. Along with aromatase, 5 beta-reductase was expressed at high levels in these mixed cell cultures, including cultures highly enriched in glia. This suggests that 5 beta-reductase is present in non-neuronal cells in brain, possibly co-localized with aromatase. However, despite the presence of 5 beta-reductase, aromatase was detected reliably in vitro even when the concentration of substrate was low. Thus, 5 beta-reductase does not prevent the synthesis of estrogen in the telencephalon of developing zebra finches. By contrast, 5 alpha-reductase activity was very low or absent in these cultures.(ABSTRACT TRUNCATED AT 250 WORDS)